Fixed #22992 -- regression in .filter(generic_fk=...) error message

Generic Foreign Keys can't be used as lhs in lookups for historical
reasons. Django 1.6 gave a FieldDoesNotExist exception when using GFKs
as lhs in lookups, but due to regression caused by lookup refactor patch
(20bab2cf) the exception type was
changed to AttributeError.

It might be a good idea to add support for gfk__exact and gfk__in lookups
later on.

Thanks to glicerinu@gmail.com for the report. The code in this commit
was written by Tim Graham.